1. Field of the Invention
The present invention relates to methods of denitrating exhaust gas for removing nitrogen oxides (NOx) contained in the exhaust gas using ammonia reducing agents and denitrating catalysts, and particularly to a method of denitrating exhaust gas, which can substantially suppress the release of ammonia.
2. Description of the Related Art
As a method of removing NOx contained in exhaust gas, selective catalytic reduction using ammonia as the reducing agent is practically used, especially for thermal electric power plants. As the catalyst, titanium oxide type catalysts containing vanadium, tungsten or molybdenum as active ingredient are used predominantly.
Recently the release of NOx have been regulated increasingly strictly, and especially in large city areas, regulations on the total amount of exhaust gas are carried into effect. Accordingly, for the power plants located adjacent urban areas, a higher effective denitration process is required for new power generating systems being built to accommodate the increasing demands for electric power.
Generally, the conventional denitration method is catalytic reduction using ammonia as a reducing agent for decomposing NOx into nitrogen (N2) in the presence of a catalyst in accordance with the following chemical formula:
4NO+4NH3+O2xe2x86x924N2+6H2O
From the formula above, it can be considered that NOx could be removed 100% theoretically by adding NH3 in the equal mole amount to NOx. Actually, however, it is impossible to blend NH3 completely uniformly with NOx in the exhaust gas.
Thus, it is needed to add excessive NH3 to NOx to achieve highly effective denitration. As a result, unreacted NH3 (it is also referred to as unreacted ammonia) remains in the exhaust gas in a considerably high proportion.
The inventors have proposed, in Japanese Patent Unexamined Publication No. 8-38856(1996), oxidative decomposition of exhaust gas into nitrogen and nitrogen oxides by introducing the exhaust gas into an ammonia decomposing catalyst layer after the ammonia denitrating process described above. In this method, however, it may be difficult to control the denitration ratio of exhaust gas at 90% or higher while keeping the exhaust of unreacted ammonia at 0.1 ppm or less.
In light of the foregoing prior art, it is an object of the present invention to provide a method of denitrating exhaust gas, which can control the NOx removal efficiency (or denitration efficiency) of exhaust gas at 90% or higher while suppressing exhaust of unreacted ammonia to 0.1 ppm or less.
The object described above is achieved by the following aspects of the present invention.
(1) A method of denitrating exhaust gas for removing nitrogen oxides by catalytic reduction, in which a nitrogen-oxide-containing exhaust gas is introduced into a catalyst-filled reaction chamber and ammonia is added in an excessive amount as reducing agent, the method comprising the steps of: introducing exhaust gas and ammonia into a first denitrating catalyst layer disposed on an upstream side in the gas stream to remove nitrogen oxides; supplying the gas into a first ammonia decomposing catalyst layer disposed on a downstream side from the first denitrating catalyst layer to control the ammonia content to be suitable for the subsequent denitrating process, the first ammonia decomposing catalyst layer having ability to oxidatively decompose unreacted ammonia into nitrogen and nitrogen oxides; then supplying the gas into a second denitrating catalyst layer disposed on a downstream side of the first ammonia decomposing catalyst layer; and supplying the gas into a second ammonia decomposing catalyst layer disposed on a further downstream side to remove the residual ammonia, the second ammonia decomposing catalyst layer being provided with a catalyst which can oxidatively decompose the residual ammonia into nitrogen and nitrogen oxides or a catalyst which can oxidatively decompose the residual ammonia into nitrogen oxides.
(2) A method of denitrating exhaust gas according to (1), wherein the catalyst used in the first ammonia decomposing catalyst layer provides nitrogen selectivity exceeding 70%, wherein the nitrogen selectivity being defined by the following equation:
Nitrogen selectivity (%)=[1xe2x88x92{CNOxOUTxe2x88x92CNOxIN}/{CNH3INxe2x88x92CNH3OUT}]xc3x97100,
wherein
CNOxOUT: NOx(ppm) at the outlet of the ammonia decomposing catalyst layer,
CNOxIN: NOx(ppm) at the inlet of the ammonia decomposing catalyst layer,
CNH3OUT: NH3(ppm) at the outlet of the ammonia decomposing catalyst layer, and
CNH3IN: NH3(ppm) at the inlet of the ammonia decomposing catalyst layer.
(3) A method of denitrating exhaust gas according to (1), wherein the catalyst used in the first ammonia decomposing catalyst layer has, in its dehydrated form, a chemical formula expressed by (1.0xc2x10.6)R2O.[aM2O3.bAl2O3].cMeO.ySiO2(R: alkali metal ion and/or hydrogen ion; M: one or more elements selected from the group consisting of Group VIII elements in the periodic table, rare earth element, titanium, vanadium, chromium, niobium, antimony and gallium; Me: alkaline earth metal element; and a+b=1, axe2x89xa70, bxe2x89xa70, cxe2x89xa70, y/c greater than 12 and y greater than 12), and contains a crystalline silicate as catalyst support, the crystalline silicate having an X-ray diffraction pattern shown in Table 1 which will be described herein below, and one or more metals selected from the group consisting of platinum, palladium, ruthenium and iridium as active metal.
(4) A method of denitrating exhaust gas according to (1) above, wherein the catalyst used in the second ammonia decomposing catalyst layer is the catalyst described in (2) or (3) above, or contains one or more oxides selected from the group consisting of silica, alumina, titania and zirconia as catalyst support, and one or more metals selected from the group consisting of platinum, rhodium, palladium, ruthenium and iridium as active metal.
Utilization of those aspects of the present invention leads to highly efficient NOx removal of exhaust gas while substantially suppressing the release of ammonia which is used as reducing agent.